Colorimetric dyes are among the oldest chemical detection technologies, in use for at least 350 years and perhaps even since antiquity. See A. A. Baker, A History of Indicators, Chymia 9 (1964) 147-167. Sensors based on colorimetric detection are commercially available for a wide range of compounds and conditions. Because of their simple visual readout, detection systems based on colorimetric indicators can be very inexpensive and are thus widely deployed. For example, M8 and M9 colorimetric papers remain, over 45 years after their initial deployment, the most widely deployed detectors of chemical warfare agents (CWA) in the United States Military. See J. K. Smart, History of Chemical and Biological Detectors, Alarms, and Warning Systems, United States Army Soldier and Biological Chemical Command, Aberdeen, Md., 2000; Field Manual 3-3 Chemical and Biological Contamination Avoidance, United States Army, 1992.
The readout of colorimetric sensors is typically done by an operator in close proximity to the sensor who measures the color change by eye. In the case of detectors for hazardous materials, this presents difficulty for the operator who must physically enter the hazard zone. This requires personnel with special training, cumbersome personal protective equipment, and provision for decontamination upon return from the hazard zone. All of these preparations add up to turn an inexpensive and rapid sensor into a time consuming and expensive sensing system. Even if hazardous materials are not involved, reading out a large number of colorimetric sensors, for example to map the extent of contamination, is time consuming as the operator must physically approach each one. Proximity readout also presents problems for sensors in remote locations, for example monitoring water quality in remote lakes, where the difficulty of access renders visiting the sensors for readout impractical.
Discrete retroreflectors (i.e. not retroreflective tapes) have been used as part of a spectrometer for remote sensing of atmospheric vapors, transforming an otherwise bistatic measurement of the atmosphere into a monostatic one. See Z. Bacsik, J. Mink, and G. Keresztury, FTIR spectroscopy of the atmosphere. I. Principles and methods, Appl. Spectrosc. Rev. 39 (2004) 295-363. A sensor that uses optical changes in the path to a retroreflector is described in U.S. Pat. No. 6,668,104 to Mueller-Fiedler et al. A chemical sensor for remote interrogation having a surface relief structure, e.g., a surface grating, positioned over a retroreflector is disclosed in U.S. Pat. No. 7,262,856 to Hobbs et al. U.S. Pat. No. 5,822,074 to V. A. Deason et al. describes using a retroreflector that is partially isolated from the environment for remote readout of colorimetric detectors. There, a user must know where the isolated portion lies in order to compare that portion with the exposed portion. Unfortunately, this limits such a device to uses where the orientation of the retroreflector is known.